The present invention relates to a process for reducing the nitrogen oxides content of the exhaust gas from an internal combustion engine by storing the nitrogen oxides in the form of nitrates on a storage catalyst during phases when the engine operates with a greater than stoichiometric air/fuel ratio, decomposing the nitrates to give nitrogen oxides during phases when the engine operates with a less than stoichiometric air/fuel ratio, and reducing the nitrogen oxides which are released.
Processes like this are used to treat the exhaust gases from internal combustion engines, which are mainly operated with a greater than stoichiometric air/fuel ratio. These are diesel engines and lean burn petrol engines, in particular petrol engines with direct injection of the petrol.
The air/fuel ratio for stoichiometric combustion depends on the composition of the fuel. Conventional engine fuels have a stoichiometric air/fuel ratio of about 14.6, that is 14.6 kg of air are required for complete combustion of 1 kg of fuel. The internal combustion engines mentioned above are operated with air/fuel ratios of greater than 16 during most of their operating period. The so-called "normalized air/fuel ratio" or ".lambda." is often used to describe the composition of the exhaust gas instead of the air/fuel ratio. This ratio .lambda. is the air/fuel ratio normalized to the stoichiometric ratio. An exhaust gas with a stoichiometric composition, therefore, has a normalized air/fuel ratio of 1. Rich exhaust gas compositions have values less than 1, and lean exhaust gas compositions have values greater than 1.
The harmful substances in the exhaust gas from these engines consist substantially of carbon monoxide, unburnt hydrocarbons, and nitrogen oxides. The nitrogen oxides consist mainly of nitrogen monoxide and nitrogen dioxide, wherein nitrogen monoxide, depending on the operating phase of the engine, makes up the major component of about 90 vol. %. As a result of operating with an above stoichiometric composition, the exhaust gas contains a high proportion of oxygen, generally more than 6 vol. %. In addition, the exhaust gas also contains about 10 to 15 vol. % of water.
Due to the high oxygen content, the oxidizable constituents of the exhaust gas (carbon monoxide and hydrocarbons) can relatively easily be converted to harmless water and carbon dioxide using so-called oxidation catalysts. In contrast, reduction of the nitrogen oxides presents great difficulties due to the high oxygen content and the low selectivity of the reaction of nitrogen oxides with the hydrocarbons and carbon monoxide acting as reducing agents.
One possibility, converting the nitrogen oxides in the oxygen-rich exhaust gas to harmless nitrogen, comprises using the process described at the beginning. An essential component of the process is a nitrogen oxides storage catalyst, or a "storage catalyst," for short. This storage catalyst consists mainly of a basic storage material, generally alkali metal or alkaline earth metal oxides, and a catalytically active noble metal from the platinum group of metals (i.e., ruthenium, rhodium, palladium, osmium, iridium, and platinum). Platinum is preferably used, this metal having the highest activity of all the noble metals for the production of NO.sub.2.
During above stoichiometric operation of the internal combustion engine, the nitrogen monoxide contained in the exhaust gas is oxidized to nitrogen dioxide by the noble metal in the catalyst and is stored in the form of nitrates by the basic storage material. The rate of absorption of nitrogen oxides decreases with time due to saturation of the storage material, so a regeneration procedure has to be initiated. For this purpose, the engine is switched to below stoichiometric operation (enrichment of the fuel/air mixture) for a brief period by the engine electronics system, which produces an overall reducing gas atmosphere over the nitrogen oxides storage catalyst. The nitrates are then decomposed to give nitrogen oxides, which are desorbed from the storage material and converted into harmless nitrogen on the noble metal component of the catalyst, with the assistance of the carbon monoxide, hydrocarbons, and hydrogen, which are then present in excess in the exhaust gas. Suitable storage catalysts are described, for example, in EP 0 562 516 A1, which document is entirely incorporated herein by reference. A mixed oxide of barium oxide and lanthanum oxide is used as the storage material in this patent application. The catalytically active component in this case is platinum.
In addition to the technique using storage catalysts described above, plasma-supported treatment processes have been developed recently. Along these lines, EP 0 659 465 A2 describes a process for treating exhaust gases in which the gas is subjected to an electric gas discharge and is contacted with catalytic material. The gas discharge preferably takes place in a region that contains catalytic material. EP 0 659 465 A2 is entirely incorporated herein by reference.
EP 0 736 320 A1 describes a process for the continuous removal of nitrogen oxides in exhaust gases from internal combustion engines with excess oxygen, wherein a reactive, nitrogen-containing plasma-jet is injected into the exhaust gas stream. The plasma-jet is produced by high frequency electromagnetic fields. EP 0 736 320 A1 also is entirely incorporated herein by reference.
Another process for treating exhaust gases with the assistance of electric discharges is described in WO 96/37690, which document is entirely incorporated herein by reference. In this process, the exhaust gas flows through a discharge chamber with an electric field in which dielectric discharges (barrier discharges) are produced.
WO 97/03746 discloses a process for the plasma-chemical decomposition and/or destruction of harmful substances, wherein the harmful substances, such as an exhaust gas stream, are passed through a section in a reactor volume that is subject to dielectrically impeded ("silent") discharges. Discharge takes place in a three-dimensional structure in which the entire reactor volume is divided in the axial direction into discharge zones and discharge-free zones, wherein local field peaks are present in the discharge zones, which leads to an increase in the effective electronic energy during discharge. WO 97/03746 also is entirely incorporated herein by reference.
DE 195 10 804 A1 describes a process for nitrogen oxides reduction in exhaust gases from internal combustion engines with excess oxygen, wherein the exhaust gases are brought into contact with a catalyst while introducing a selective gaseous reducing agent. In the process described in this patent document, the reducing agent is converted into a high pressure plasma state with extensive radical production before entering into contact on the catalyst, so that the reaction is accelerated. DE 195 10 804 A1 is entirely incorporated herein by reference.
WO 98/00221 describes an exhaust gas treatment unit including a storage device for nitrogen oxides, hydrocarbon, and particle emissions from an internal combustion process, and a plasma reactor for decomposing the stored emissions. The storage device may be arranged completely upstream from or overlapping with the plasma reactor or fully inside the plasma reactor. The harmful substance emissions mentioned above are collected for a certain period and then decomposed in the plasma reactor. The plasma reactor is operated only during the release phase for the harmful substances in order to avoid the production of unwanted nitrogen dioxide and nitric acid and to minimize the energy consumption associated with operating the plasma reactor. WO 98/00221 also is entirely incorporated herein by reference.
The processes described for the treatment of exhaust gases sometimes have considerable problems and shortcomings. For example, in the case of a storage catalyst, the surface stability of the platinum used presents problems after high temperature ageing, in particular in a lean exhaust gas. In addition, oxidation of nitrogen monoxide to nitrogen dioxide starts only in the temperature range between 150 and 200.degree. C. and shifts to higher temperatures after thermal ageing of the catalyst. A reasonable conversion of nitrogen oxides with the assistance of a storage catalyst is possible only to a limited extent with diesel vehicles at the low exhaust gas temperatures that are produced under urban driving conditions.
The purification processes using electric gas discharges discussed above also have not hitherto led to the desired result. Measurements of the effects of an electric gas discharge on the oxidized nitrogen components in a nitrogen oxides-containing lean exhaust gas show, in fact, that although a high proportion of nitrogen monoxide is reduced to almost zero by the discharge, higher oxidized nitrogen components are produced, such as nitrogen dioxide and also, due to reaction with the water in the exhaust gas, nitric acid. An overall reduction in the oxidized nitrogen components cannot therefore be observed.